[theora-dev] newest cvs and mplayer
giles at xiph.org
Thu Jun 5 15:20:17 PDT 2003
On Thursday, June 5, 2003, at 09:54 pm, David Kuehling wrote:
> What is the cleanest way to detect header packets? For vorbis that was
> done by checking some bits in the packet, but that method failed for
> theora. I currently don't have the time to do read all the docs and
> sources, so I'd be very pleased for helpful comments...
the header packets all begin with 0x8*, 'theora'. That is, the first
bit is '1' for non-data packets. 0x80 is the first header, 0x81 are the
comments and 0x82 are the decoder tables. Data packets begin with the
> Another problem is the detection of keyframes which I implemented using
> some kind of stupid "heuristic". Someone on mplayer-dev told me that
> should break when bitstream changes, but it still works so far. So I'm
> not very motivated to change it as long as theora bitstream isn't
There's a hack with the granulepos to assist seeking; the granulepos is
really a fixed point number, with the integer part counting keyframes
and the fractional part counting frames since the last keyframe.
Therefore you can look for the lower-order bits being zero to identify
a keyframe. The radix is variable from file to file however, you'll
have to pull it out of the header to be reliable.
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